The primary objective of the Colorado Adoption Project--a longitudinal, prospective adoption study funded by NICHD since 1977-- is to assess genetic and environmental influences on individual differences in behavioral development among children. Biological parents who relinquished their children for adoption at birth, parents who adopted these children, matched "control" (nonadoptive) parents, and the children (probands and younger siblings) in these families are studied. Adults were administered a 3-hour battery of behavioral tests that included measures of the major factors of cognitive abilities and personality; information pertaining to family background, common medical and behavioral problems, interests and talents, and frequently used drugs has also been obtained. For the children, the assessments employ standard tests of cognitive and language development, personality/temperament, motor development, and health. Environmental assessments in the adoptive and control homes are emphasized. The adopted and control probands and their younger siblings are emphasized. The adopted and control probands and their younger siblings are studied in their homes at 1, 2, 3, and 4; in the laboratory at 7 and 12; and via telephone interviews at 9, 10, and 11. Testing at 13 through 16 years is planned. The proposed continuation of HD18426 for a final 5-year period will allow us to complete testing of 624 children in the CAP when they are 7 years old. During the renewal period, 214 children will be tested. Children have already been tested at ages 1, 2, 3, and 4 years, and data collection during adolescence is currently in progress. Completion of data collection at 7 years of age is essential to bridge the developmental epochs of early childhood and adolescence. Of the various methods available to study the origins of individual differences in behavioral development, the full adoption designed employed in the Colorado Adoption Project is the most powerful. The proposed continuation of HD18426 is a small, but important, step toward completion of this landmark study, a study that will substantially advance basic research in child development and will have important implications for child-rearing practices, education, and mental health.